Bacterial vaginosis (BV) results from a shift in the microbial ecosystem of the vagina from Lactobacillus predominance to overgrowth by anaerobic and facultative species, and has been associated with postpartum/postabortal endometritis, preterm birth, pelvic inflammatory disease, and human immunodeficiency virus acquisition. The etiology of BV is unclear, as is the role of sexual transmission of an undefined precipitant. BV frequently recurs in women who initially respond to standard antibiotic therapy. More effective interventions to prevent and treat BV require an understanding of the role of sexual transmission. Relative to most heterosexual women, lesbians have a two to three-fold higher BV prevalence (25 percent-52 percent). Preliminary evidence strongly implicates sexual transmission of vaginal secretions between women as a risk for BV. The proposed work will define the temporal association between sexual practices that transfer vaginal secretions and BV acquisition, and design an intervention to prevent this transfer and test its efficacy in reducing BV recurrence. Specific aims are: (1) prospectively define risk factors associated with acquisition of BV in a cohort of lesbians, including sexual practices that transfer vaginal secretions, sex with men, lubricant use, douching, menses, and changes in vaginal lactobacilli. The hypothesis is that BV in lesbians occurs after sexual transmission of vaginal fluid from a woman with BV to a woman without BV; that women not colonized with vaginal hydrogen peroxide-producing lactobacilli will be at highest risk for BV acquisition by this mechanism; and that comparative analyses of vaginal flora in sex partners will show similar microbial profiles. (2) Test the efficacy of an intervention to reduce transfer of vaginal fluid between female sex partners in reducing recurrence of BV following treatment with metronidazole in a prospective, randomized trial. The hypothesis is that the intervention will improve knowledge, attitudes, beliefs, and intention about BV prevention, reduce sexual exposures that increase risk of transfer of vaginal fluid, and reduce rates of BV recurrence. Lesbian couples provide a unique opportunity to conduct comparative studies of vaginal microbial ecology in sex partners, and to directly analyze determinants of transmission.